When a machine with a hydrostatic transmission is in neutral on slopes, there is a tendency for it to ‘drift’ down the hill due to hydraulic leakage in the system. This leakage allows for a non-zero flow across the motor when the pump displacement is commanded to neutral. To minimize the drift of a machine with a hydrostatic transmission, operators tend to use dynamic braking by applying the static parking brake while the machine is drifting. This stops the drift but shortens the life span of the brakes.
Alternative hydraulic systems, such as open loop transmissions, often utilize counterbalance valves incorporated directly inside the track motor to minimize drift. Counterbalance valves accomplish this feature by maintaining a differential pressure on the travel motors with minimal leakage and therefore prevent noticeable motion or drift. This is generally a hydraulic solution and does not utilize electronic control. Counterbalance valves are undesirable in hydrostatic systems because of the added pressure drop and efficiency loss. Open loop systems can also introduce multifunction interference issues when multiple hydraulic systems request more flow than the pump for the open loop system can provide. Typically closed loop, hydrostatic systems are utilized where increased tractive effort and efficiency is a priority.
It would be desirable to have a system that can electronically hold a machine with a hydrostatic transmission at zero speed when the transmission is in neutral on slopes so that the machine does not drift. Being able to drive machines to zero speed on slopes can also reduce the need for dynamic braking on the machines (applying the static park brake while the machine is drifting) which will improve the brake life of the machines.